I could write novels about this, which i did in my early days here...
For now, i'll settle with:
1. He is without a doubt the character with the most development in the entire manga. A sweet little kid aspiring to surpass his brother, a standoffish but caring boy, a grown teenager hellbent on revenge, a drowned in darkness maniac, a person that continues to seek answers, a resolute revolutionist, and finally - his true self.
2. Sasuke was never a one-sided person and yet was the most focused on his goals character. His objectives was always what drove him, but he also had a ''second layer'' that defined him. The best example is him repeating that he'd do anything to stay alive and kill Itachi, yet was ready to die for Naruto and Sakura without a second thought.
3. He is a realist. He's the classic ''results trump the means needed to achieve them'' person, something with which i strongly agree with. His mindset allows him to look at the bigger picture instead of relying on pipe dreams.
4. Sasuke does things his own way. He refused to kill Naruto for MS, he doesn't do missions, doesn't capture Bijus, isn't bound for long by any common objective of a faction he's been in - Konoha, Sound 4/Orochimaru, Akatsuki. He made his own team, reminiscent of his previous one (making it all the better), and started following following his path.
5. A complete badass that doesn't take sh*t from anybody.